Fight or Flight: How the Nose Knows What to Do

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Cats arch their backs at the smell of a rival, and mice scurry at
the scent of a fox. But how does the nose know who or what is
lurking? Now scientists have identified several special receptors
in the noses of animals that react to specific scents given off
by others.

It's these receptors that signal to the brain whether the animal
needs to flee, make itself large and scary, or perhaps even woo a
mate.

"Animals in the wild need to be
able to recognize other animals, whether they are predators,
potential mates or rivals," study researcher Catherine Dulac of
Harvard University told LiveScience. "Many animals rely on the
sense of smell; they can distinguish one type of encounter from
another one based on chemicals."

Experimenting on mice, Dulac and her fellow researchers
discovered that more of the animal's receptors seem to be
dedicated to sniffing out predators than to detecting potential
mates.

Animal senses

When a cat or mouse senses the chemical compounds secreted by
other animals, it activates a special sensor in the nose called
the
vomeronasal organ. This organ, which is found in many animals
and consists of a set of receptors, sends a signal to the brain,
which interprets the signal and takes action. (Though humans have
lost this organ, research has suggested humans do react in
various ways to chemical cues.)

The first receptor genes were discovered more than 15 years ago
by Dulac herself, but researchers are just now figuring out which
receptors respond to what kinds of chemical signals. "From a
given receptor gene, there is no information about what the
receptor protein detects," Dulac said. "You have a bunch of
receptors and you didn't know what they do."

The researchers set out to determine what chemicals these animals
might be recognizing and how specialized these receptors are.

The researchers ran their experiments on mice, using a special
method to identify the receptors in the nose that are associated
with different vomeronasal responses and consequent behaviors.
They marked the rodents' vomeronasal neurons with a compound that
would make the neurons glow when activated. They also looked at
which neurons expressed which receptors.

The researchers watched which brain cells lit up in response to a
particular chemical signal, then determined which receptors the
cells had in common, pinpointing which could be reacting with the
signal.

Predator or potential mate?

Animals rely on these chemicals secreted by others to identify
both
individuals of their own species (in this case, they are
often called pheromones) and those of other species. Dulac said
that to the researchers' surprise, most of the receptors they
found were cued to respond to scents from other species,
especially those from potential predators.

"The vomeronasal organ has always been advertised as the organ
that determines
mating behaviors," Dulac said. "We expected we could find
male-specific cues and female-specific cues, but actually most
are responsive to other animals."

The mice have behavioral responses to these different chemicals.
If they sense the chemicals of a predator, like a fox, they will
quiver in the corner of their cages. "If the animal needs to
recognize every possible predator ... then you probably need a
lot of receptors just to be safe," Dulac said. "Some are
specialized in detecting predators; some, even the type of
predator — reptilian, mammalian, ferret or fox."

A human's vomeronasal organ seems to disappear during fetal
development. The
presence of human pheromones is still hotly debated, though.
Some studies argue that pheromone-like chemical-behavior
interactions could come from normal scent-based communications in
humans.

The study was published today (Sept 21) in the journal Nature.

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